Melt out metal cores of complex shapes are made for use as cores in subsequently molded plastic components. The cores are made of metal alloy or other suitable material having a low melting temperature. They are placed in molds for making undercut hollow plastic components and then subsequently removed from the plastic components by melting the cores and leaving the undercut or hollow plastic components. The melting temperature of the metal alloy or other material is lower than that of the plastic component. In other embodiments metal alloys with low melting temperatures are used for encapsulating components such as turbine blades so they may be held for machining in other finishing steps. After use the metal from the cores or encapsulations is remelted and reused. One example of an apparatus for casting metal alloys with low melting temperatures is disclosed in U.S. Pat. No. 4,676,296. In this patent, molten metal alloy is injected by a piston moving downwards in a cylinder placed within a tank of molten metal alloy. The liquid metal alloy passes through a passageway from the bottom of the cylinder into a mold or die.
The casting of metal alloys with low melting temperatures is not similar to die casting. Die casting occurs at high pressures and dies are filled in a very short period of time. In the case of producing metal cores or encapsulations, it is necessary to allow the liquid metal alloy to flow substantially under no pressure into the mold or die. If pressure is used then porosity can occur in the casting which is unacceptable. The time to fill a mold or die is far longer than for die casting. Thus, it is apparent that controlling the flow of metal alloy into a mold or die is critical.
In our co-pending application Ser. No. 268,492 filed Nov. 8, 1988, and now U.S. Pat. No. 4,958,675, a metal casting process is disclosed wherein the injection cylinder is filled with molten metal alloy from the tank through a valve port in the injection passageway leading to the injection cylinder by raising the piston in the cylinder. The system discloses a block valve outside the tank in the passageway to the die.